Backrest of a chair



FIG. 1 is an upper, front, and right side perspective view of a backrest of a chair showing the new design;

FIG. 2 is a lower, rear, and right side perspective view thereof;

FIG. 3 is a front elevation view thereof;

FIG. 4 is a rear elevation view thereof;

FIG. 5 is a left side elevation view thereof;

FIG. 6 is a right side elevation view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof. 

CLAIM The ornamental design for a backrest of a chair, as shown. 